Messages in this thread | | | Date | Mon, 11 Jan 2016 11:02:49 -0800 | From | Alexei Starovoitov <> | Subject | Re: [PATCH 5/9] bpf: syscall: add percpu version of lookup/update elem |
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On Mon, Jan 11, 2016 at 11:56:57PM +0800, Ming Lei wrote: > Prepare for supporting percpu map in the following patch. > > Now userspace can lookup/update mapped value in one specific > CPU in case of percpu map. > > Signed-off-by: Ming Lei <tom.leiming@gmail.com> ... > @@ -265,7 +272,10 @@ static int map_lookup_elem(union bpf_attr *attr) > goto free_key; > > rcu_read_lock(); > - ptr = map->ops->map_lookup_elem(map, key); > + if (!percpu) > + ptr = map->ops->map_lookup_elem(map, key); > + else > + ptr = map->ops->map_lookup_elem_percpu(map, key, attr->cpu);
I think this approach is less potent than Martin's for several reasons: - bpf program shouldn't be supplying bpf_smp_processor_id(), since it's error prone and a bit slower than doing it explicitly as in: http://patchwork.ozlabs.org/patch/564482/ although Martin's patch also needs to use this_cpu_ptr() instead of per_cpu_ptr(.., smp_processor_id());
- two new bpf helpers are not necessary in Martin's approach. regular map_lookup_elem() will work for both per-cpu maps.
- such map_lookup_elem_percpu() from syscall is not accurate. Martin's approach via smp_call_function_single() returns precise value, whereas here memcpy() will race with other cpus.
Overall I think both pre-cpu hash and per-cpu array maps are quite useful. For this particular set I would suggest to rebase on top of Martin's to reuse BPF_MAP_LOOKUP_PERCPU_ELEM command that should be applicable to both per-cpu array and per-cpu hash maps. and add BPF_MAP_UPDATE_PERCPU_ELEM via smp_call as another patch that should work for both as well.
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